It is your responsibility when preparing to determine beneficiaries of online property whether the terms of the service involved allows for such transfers.  For example, as of this time, Second Life has the following statement in Section 2.4 of its Terms of Service.

You may not transfer your Account to any third party without the prior written consent of Linden Lab; notwithstanding the foregoing, Linden Lab will not unreasonably withhold consent to the transfer of an Account in good standing by operation of valid written will to a single natural person, provided that proper notice and documentation are delivered as requested by Linden Lab. 

As you can see, it is not an issue as long as it is included in a written will.  Not all online services make such allowances.  For instance, the Terms of Service for Yahoo! accounts has the following under No Right of Survivorship and Non-Transferability:

You agree that your Yahoo! account is non-transferable and any rights to your Yahoo! ID or contents within your account terminate upon your death. Upon receipt of a copy of a death certificate, your account may be terminated and all contents therein permanently deleted.

In some cases, it may even be necessary to identify the individual(s) within the website account itself.  This is especially true of online insurance accounts.